First measurement of radioactive isotope production through cosmic-ray muon spallation in Super-Kamiokande IV

Abstract

2Cosmic-ray-muon spallation-induced radioactive isotopes with β decays are one of the major backgrounds for solar, reactor, and supernova relic neutrino experiments. Unlike in scintillator, production yields for cosmogenic backgrounds in water have not been exclusively measured before, yet they are becoming more and more important in next generation neutrino experiments designed to search for rare signals. We have analyzed the low-energy trigger data collected at Super-Kamiokande-IV in order to determine the production rates of 12 B, 12 N, 16 N, 11 Be, 9 Li, 8 He, 9 C, 8 Li, 8 B and 15 C. These rates were extracted from fits to time differences between parent muons and subsequent daughter β's by fixing the known isotope lifetimes. Since 9 Li can fake an inverse-beta-decay reaction chain via a β + n cascade decay, producing an irreducible background with detected energy up to a dozen MeV, a dedicated study is needed for evaluating its impact on future measurements; the application of a neutron tagging technique using correlated triggers was found to improve this 9 Li measurement. The measured yields were generally found to be comparable with theoretical calculations, except the cases of the isotopes 8 Li/ 8 B and 9 Li.

Developing the MeV potential of DUNE: Detailed considerations of muon-induced spallation and other backgrounds

“…The precision of predicting isotope yields, which is mostly limited by the uncertainties in hadronic processes, is typically a factor of ≈ 2. For example, in Super-Kamiokande [33], the FLUKA-predicted yields of some isotopes agree with measurements within a few tens of percent; some are off by a factor ≈ 2-3. In Borexino [34], FLUKA predictions also agree well with experimental measurements.…”

“…For half of them, including some important ones such as 40 Cl, 38 Cl, 34 P, etc., we find a good agreement, within a factor of ≈ 2. For some rarely produced isotopes ( 37 P, 33 Cl, 35 Ar, etc. ), our yields are a factor ≈ 0.1 of theirs.…”

Search for proton decay via p→e+π0 and p→μ+Abe1, Haga2, Hayato3 et al. 2017Phys. Rev. DSelf Cite19572090View full textAdd to dashboardCite

“…Furthermore, these proton decay modes do not produce neutrons in secondary interactions in water because the final state particles in these decay modes are lepton and gammas. The neutron tagging algorithm was originally developed to identify anti-neutrino interactions, in which a neutron is often emitted; the supernova relic neutrino search [16], the cosmic-ray-muon spallation background measurement [17], and the neutrino oscillation analysis [18] have been improved by this technique. To find 2.2 MeV γ candidates, we search for hit clusters with ≥ 7 hits within a 10 ns sliding time window, after time-of-flight (TOF) subtraction using the vertex of the prompt neutrino interaction.…”

Inclusive nucleon decay searches as a frontier of baryon number violation

“…A kicked-out neutron will quickly thermalize and diffuse, subsequently being captured on a hydrogen atom with an accompanying emission of a low-energy 2.2 MeV γ-ray. A low-efficiency neutron tagging is currently being used in SK to gather this signal [101], which is below the energy threshold of SK. The planned SK upgrade involving gadolinium doping [102] will allow to detect neutrons with high efficiency due to ∼ 8 MeV γ-ray emission accompanying neutron capture on gadolinium.…”